Polyaxial orthopedic fastening apparatus with independent locking modes

ABSTRACT

An apparatus is designed to attach an implant to bone in a manner that permits rotational adjustment of the implant about multiple axes prior to securement via the apparatus. The apparatus includes separate rotational and translational fasteners that can be individually locked to independently restrict rotation and translation of the implant relative to the bone. The rotational fastener includes an interpositional member, an expandable engagement member, and a rotational locking member that urges the expandable engagement member to advance along the interpositional member. The resulting expansion of the engagement member causes it to engage the implant. The rotational fastener is slidable along a fixation member implanted in the bone until the translational fastener is applied to restrict relative translation between the rotational fastener and the bone.

CROSS REFERENCE TO RELATED APPLICATIONS

This Patent Application is a continuation application of U.S. Ser. No.14/551,305, filed Nov. 24, 2014, which is a continuation of U.S. patentapplication Ser. No. 13/270,599, filed Oct. 11, 2011, now issued as U.S.Pat. No. 8,920,468, which is a continuation of U.S. patent applicationSer. No. 11/312,323, filed on Dec. 19, 2005, now issued as U.S. Pat. No.8,062,336, which is a continuation-in-part of U.S. patent applicationSer. No. 11/063,941, filed on Feb. 22, 2005, now issued as U.S. Pat. No.7,993,373, the entire contents of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates generally to systems and methods forattaching implants to bone, and more specifically, to a polyaxialorthopedic fastening apparatus particularly useful in the field of facetjoint replacement.

2. The Relevant Technology

Orthopedic medicine provides a wide array of implants that can beattached to bone to alleviate various pathologies. One unique challengein orthopedics is to provide implants and fastening devices that areadaptable to a variety of bone morphologies. Each patient will have adifferent bone structure; accordingly, it may be necessary to allow foradjustable positioning of an implant with respect to the bone so thatthe implant will be positioned to perform its function.

For this reason, a number of fixation systems have been invented thatenable variation of the angle between the implant and the fastener.Although such fixation systems generally permit adaptation to the bonemorphology of a patient to provide secure anchoring of the implant tobone, they are generally somewhat limited in the types of adjustmentthey permit with respect to the bone. Accordingly, such fixation systemsmay not be usable with a number of implants that require morecomprehensive adjustability. Furthermore, many known implant fixationsystems are complex due to the presence of several parts, or due to theneed to perform several steps to utilize them to attach an implant tobone. Yet further, some known implant fixation systems are expensive,and require the use of unusual tooling. A need exists in the art forimplant fixation systems and methods that alleviate the foregoingshortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be discussed withreference to the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope.

FIG. 1 is an exploded, perspective view of a vertebra with an apparatusaccording to one embodiment of the invention, with the apparatuspositioned to attach an implant to the vertebra.

FIG. 2 is a perspective view of the vertebra with the apparatus of FIG.1 secured to the vertebra in the locked configuration to lock bothrotation and translation of the implant.

FIG. 3 is a cephalad, section view of the vertebra with the apparatus ofFIG. 1 secured to the vertebra in the locked configuration as in FIG. 2.

DETAILED DESCRIPTION

The present invention advances the state of the art by providing systemsand methods that can be used to anchor orthopedic implants to bone in amanner that provides a high degree of implant adjustability, simplicity,and ease of use. The present invention can be used in any orthopedicprocedure, but may have particular utility in the field of facet jointreplacement to alleviate back pain resulting from traumatic,inflammatory, metabolic, synovial, neoplastic and degenerative spinaldisorders. The configuration and operation of selected embodiments ofthe invention will be shown and described in greater detail withreference to FIGS. 1 through 4, as follows.

In this application, the terms “fastener,” “interpositional member,” and“engagement member” are used broadly. A “fastener” generally relates toone or more members that can be used to “lock” two other objectstogether by restricting relative rotation and/or translation about oralong at least one axis. More precisely, a “rotational fastener” is afastener that restricts relative rotation of the two objects. A“translational” fastener is a fastener that restricts relativetranslation of the two objects. An “interpositional member” generally isa member, at least part of which is designed to be positioned between atleast two other members of a system. An “engagement member” is a memberthat is movable into and/or out of contact with another member toaccomplish a function such as locking the members together.

“Polyaxial” rotation is rotation that can occur about at least two axesthat are not parallel to each other. “Triaxial rotation” is rotationabout three perpendicular axes. Triaxial rotation is equivalent torotation about a point, because free rotation about any axis of a 3Dcoordinate system is the same as rotation that is not limited to anyaxis in the system.

Referring to FIG. 1, a perspective view illustrates an apparatus 10according to one embodiment of the invention, in use with a vertebra 12,such as an L 4 lumbar vertebra of a human spine. As shown, the vertebra12 has a body 18, which is generally disc-shaped. The vertebra 12 alsohas two pedicles 20 extending from the body 18, and a posterior arch, orlamina 22, which extends between the posterior ends of the pedicles 20to couple the pedicles 20 together. The vertebra 12 also has a pair oftransverse processes 24 that extend laterally from the pedicles 20, anda spinous process 26 that extends posteriorly from the lamina 22.

The vertebra 12 also has a pair of superior facets 28, which arepositioned toward the top of the vertebra 12 and face generallymedially. Additionally, the vertebra 12 has an inferior facet 30, whichis positioned toward the bottom of the vertebra 12 and faces generallylaterally. A resected inferior facet 31 also faces generally laterally.The articular surface of the resected inferior facet 31 may optionallyhave been resected away to prepare the resected inferior facet 31 forarthroplasty. Each of the pedicles 20 of the vertebra 12 has a saddlepoint 32, which is positioned generally at the center of the juncture ofeach superior facet 28 with the adjacent transverse process 24.

The superior facets 28 of the vertebra 12 articulate (i.e., slide and/orpress) against the inferior facets (not shown) of an adjacent superiorvertebra (not shown), such as an L 3 lumbar vertebra, to limit relativemotion between the vertebra 12 and the superior vertebra. Thus, thecombination of each superior facet 28 with the adjacent inferior facetdefines a facet joint (not shown). Accordingly, two facet joints spanthe distance between each adjacent pair of vertebrae. The inferiorfacets 30 of the vertebra 30 are part of other facet joints that controlmotion between the vertebra 12 and an adjacent inferior vertebra (notshown), such as an L 5 lumbar vertebra or the sacrum.

Each of the facet joints may be covered by a capsule (not shown)containing a fluid (not shown) that reduces wear of the facets 28, 30and facilitates articulation. Additionally, layers of cartilage (notshown) may cover the facets 28, 30 to further reduce wear and facilitatearticulation. These anatomical structures, as well as the variousmuscles, ligaments, and nerves of the spine, will not be depicted in theFigures to enhance the clarity of the disclosure. Such structures may beremoved or displaced according to known methods to provide the necessaryaccess to the vertebra 12.

As shown, a semispherical resection 34 has been formed on one of thesaddle points 32 of the vertebra 12. The semispherical resection 34 isshaped to receive an implant to replace the articular surface of one orboth of the adjacent superior and inferior facets 28, 30. Thesemispherical resection 34 permits relative rotation between the implantand the vertebra 12 about three perpendicular axes prior to fixation ofthe implant to the vertebra 12. The axes may be defined as shown byreference numerals 40, 42, and 44 in FIG. 1.

More precisely, the axes may include a first axis 40, a second axis 42,and a third axis 44. The first axis 40 is generally collinear with theaxis of the corresponding pedicle 20. The second axis 42 is generallyvertical (i.e., parallel to the axis of the body 18) and perpendicularto the first axis 40. The third axis 44 is generally horizontal (i.e.,parallel to the end plates of the body 18) and perpendicular to thefirst and second axes 40, 42.

The apparatus 10 includes an implant 50, a fixation member 52, arotational fastener 54, and a translational fastener 56. The implant 50is designed to seat against the semispherical resection 34 and toreplace the removed articular surface of the resected inferior facet 31immediately inferior to it. The fixation member 52 may take the form ofa pedicle screw designed to be implanted in the corresponding pedicle 20to anchor the implant 50 in place. The orthopedic fastener 54 isdesigned to be coupled to the fixation member 52 to hold the implantagainst the vertebra 12.

In the embodiment of FIG. 1, the implant 50 has a fixation portion 60,an articulation portion 62, and a stem 64. The fixation portion 60 isshaped to be attached to the semispherical resection 34, and thearticulation portion 62 provides a surface that articulates with anadjacent vertebral facet to carry out the function of the inferior facet30. The articulation portion 62 is coupled to the fixation portion 60 bythe stem 64.

As shown, the fixation portion 60 has a bone apposition surface 66,which may be generally semispherical to correspond to the shape of thesemispherical resection 34. The fixation portion 60 also has an aperture(not visible in FIG. 1) that passes through the bone apposition surface66 to receive the fixation member 52. The aperture is somewhat largerthan the exterior surface of the fixation member 52 so that the boneapposition surface 66 is able to slide against the semisphericalresection 34 with the fixation member 52 in place, implanted in thepedicle 20.

The articulation portion 62 similarly has an articulation surface 68designed to articulate with a superior facet of a vertebra (or sacrum)immediately inferior to the vertebra 12. The articulation surface 68 mayhave a convex shape, which may further be semispherical,semicylindrical, or the like. The articulation surface 68 may bedesigned to articulate with a natural superior facet and/or a prostheticsuperior facet.

In addition to the bone apposition surface 66, the fixation portion 60also has an engagement surface 70 shaped to receive the rotationalfastener 54 such that the rotational fastener 54 is able to restrictrelative rotation between the implant 50 and the fixation member 52. Theengagement surface 70 has a generally semispherical concave shapethrough which the aperture (not shown) of the fixation portion 60passes.

In the embodiment of FIG. 1, the fixation member 52 has a distal end(not visible in FIG. 1) 74 implanted into the corresponding pedicle 20of the vertebra 12, and a proximal end 74 that protrudes from thecorresponding saddle point 32. The distal end has threads thatfacilitate implantation of the distal end 74 in the pedicle 20 and keepthe implanted distal end in place. The fixation member 52 also has asliding interface 76 positioned between the distal end and the proximalend 76. The sliding interface 76 may have a polygonal or othernon-circular cross section shaped to receive the rotational fastener 54in such a manner that no significant relative rotation can occur betweenthe sliding interface 76 and the rotational fastener 54.

The proximal end 74 has a plurality of threads 78 that are exposed toreceive the fastener 54. Additionally, the proximal end 74 has atorquing interface 80 that may be used to apply torque to the fixationmember 52 to implant the distal end in the pedicle 20. The torquinginterface 80 may take the form of a hexagonal recess or projection thatmates with a corresponding hexagonal feature on a driver (not shown).

As shown, the rotational fastener 54 includes an interpositional member82, an engagement member 84, and a rotational locking member 86. Theinterpositional member 82 may have a generally tubular shape with atapered portion 90, a plurality of threads 92 adjacent to the taperedportion 90, and an interface 94. As shown, the tapered portion 90becomes narrower toward the threads 92. The interface 94 is designed toprovide a slidable, yet non-rotating connection between theinterpositional member 82 and the sliding interface 76 of the fixationmember 52. Accordingly, the interface 94 may take the form of a borewith a polygonal cross section that receives the corresponding polygonalcross section of the sliding interface 76 with enough clearance topermit relatively free sliding motion. Alternatively, a tighter fit maybe used to restrict sliding, but permit relative translation between theinterpositional member 82 and the fixation member 52 under theapplication of force.

As also illustrated in FIG. 1, the engagement member 84 is generallyspherical in shape, with a hollow interior. The hollow interior has ataper that generally matches the taper of the tapered portion 90 of theinterpositional member 82. The engagement member 84 has an implantengagement surface 98 with a semispherical shape, and a plurality ofgrooves 100 arranged in a parallel, substantially radially symmetricalfashion about the implant engagement surface 98. The grooves 98 permitexpansion and contraction of the implant engagement surface 98. Thehollow interior is accessible via ports 102 positioned at either end ofthe implant engagement surface 98.

The rotational locking member 86 has a bore 104 in which a plurality ofthreads 106 are formed. The threads 106 are designed to mate with thethreads 92 of the interpositional member 82. The bore 104 also has atorquing interface 108 formed therein to facilitate rotation of therotational locking member 86 into engagement with the interpositionalmember 82. The torquing interface 108 may take the form of a portion ofthe bore 104 having a generally polygonal cross sectional shape, such asa hegaxonal cross sectional shape. Thus, a corresponding hexagonalprotrusion of a driver (not shown) may be inserted into the torquinginterface 108 to rotate the rotational locking member 86 into engagementwith the interpositional member 82.

The translational fastener 56, which may also be termed a translationallocking member, has a threaded bore 112, a torquing interface 114, and aflange 116. The threads of the threaded bore 112 are sized to rotateinto engagement with the threads 78 of the proximal end 74 of thefixation member 52. The torquing interface 114 may take the form of aprotrusion having a generally hexagonal shape capable of being receivedwithin a recess of a driver (not shown) having a corresponding hexagonalshape.

The flange 116 protrudes generally radially from the exterior of thetranslational fastener 56, adjacent to the torquing interface 114. Theflange 116 may be sized to abut the adjoining annular surface of therotational locking member 86 to enable the translational fastener 56 toexert relatively uniform, linear force against the rotational lockingmember 86 upon tightening of the translational fastener 56. If desired,a portion of the translational fastener 56 may nest within the bore 104of the rotational locking member 86 to reduce the profile of theassembled apparatus 10.

The apparatus 10 may be secured to the vertebra 12 according to avariety of methods. According to one method, the fixation member 52 isfirst implanted in the corresponding pedicle 20. This may be carried by,for example, forming an incision in the overlying tissues, retractingthe tissues from the operating area, implanting a guide wire in thepedicle 20 under fluoroscopy, and then rotating the fixation member 52into engagement with the pedicle 20 through the use of a driver (notshown) coupled to the torquing interface 80.

Through the use of the rotational fastener 54 and the translationalfastener 56, the orientation of the implant 50 and the position of theimplant 50 along the fixation member 52 (i.e., along the first axis 40)may be independently locked. The rotational fastener 54 and the implant50 may first be assembled together by assembling the interpositionalmember 82, the engagement member 84, the rotational locking member 86,and the implant 50.

The engagement member 84 may first be inserted into the hollow interiorof the fixation portion 60 of the implant 50. Since the engagementmember 84 has not yet been significantly expanded, there is clearancebetween the implant engagement surface 98 of the engagement member 84and the engagement surface 70 of the fixation portion 60 of the implant50. This clearance permits rotation of the engagement member 84 withinthe fixation portion 60. The engagement surface 70 of the fixationportion 60 may have a semispherical shape that extends far enough toeffectively capture the engagement member 84. Accordingly, theengagement member 84 may need to be compacted and/or pressed into thehollow interior of the fixation portion 60.

The interpositional member 82 is then inserted through the aperture (notshown) of the fixation portion 60 and into the hollow interior of theengagement member 84 such that the tapered portion 90 extends throughthe port 102 that will be oriented toward the fixation member 52, andthe threads 92 extend through the other port 102 (i.e., the port 102that will be oriented toward the translational fastener 56 and therotational locking member 86, as shown in the exploded view of FIG. 1).Then, the rotational locking member 86 is positioned adjacent to thecorresponding port 102 such that the threads 92 enter the bore 104.

The rotational locking member 86 is rotated with respect to theinterpositional member 82 such that the threads 106 of the bore 104engage the threads 92 of the interpositional member 82. Continuedrotation of the rotational locking member 86 with respect to theinterpositional member 82 will cause the engagement member 84 to expandas the opposite port 102 slides toward the larger end of the taperedportion 90. However, at this stage, the rotational fastener 54 remainsin the unlocked configuration because the rotational locking member 86is only rotated sufficiently to engage the threads 92 to keep therotational locking member 86, the interpositional member 82, and theengagement member 84 together.

The assembled implant 50 and rotational fastener 54 may then be advancedtoward the proximal end 74 of the implanted fixation member 52 such thatthe torquing interface 80, and then at least some of the threads 78,pass through the interface 94, or bore, of the interpositional member82. The interface 94 then slides around the sliding interface 76 of theproximal end 74 of the fixation member 52. As mentioned before, someclearance may exist between the sliding interface 76 of the proximal end74 and the interface 94 of the interpositional member 82. However, thematching polygonal shapes of the sliding interface 76 and the interface94 prevent relative rotation between the fixation member 52 and therotational fastener 54.

Since the rotational fastener 54 is still in the unlocked configuration,the implant 50 may be rotated with respect to the fixation member 52 andthe vertebra 12. The implant 50 is pivoted generally about the center ofthe radius of the engagement surface 70 until the articulation surface68 is properly positioned and oriented to articulate with thecorresponding natural or prosthetic superior articulation surface. Inthe embodiment of FIG. 1, rotation of the implant 50 is not onlypolyaxial, but also triaxial. Thus, the implant 50 may be rotated aboutany axis passing through the center of the radius of the engagementsurface 70.

Once the implant 50 has been rotated into the proper orientation withrespect to the vertebra 12, it may be locked in that orientation bymoving the rotational fastener 54 to the locked configuration. Therotational locking member 86 is further rotated with respect to theinterpositional member 82, for example, by engaging the torquinginterface 108 of the bore 104 with a corresponding feature of a driver(not shown). This rotation urges the opposite port 102 to advance alongthe tapered portion 90 of the interpositional member 82, toward thelarger end of the tapered portion 90. The outward pressure on the port102 causes the engagement member 84 to expand, thereby increasing theoverall radius of the implant engagement surface 98. The implantengagement surface 98 engages the engagement surface 70 of the fixationportion 60 of the implant and exerts outward pressure on the engagementsurface 70. As a result, the implant 50 becomes locked to the engagementmember 84.

Thus, the rotational fastener 54 has reached the locked configuration,and the implant 50 is no longer rotatable with respect to the vertebra12. However, the implant 50, together with the rotational fastener 54that is now rigidly locked to it, may still move along the fixationmember 52. The translational fastener 56 may then be applied to restrictsuch translational motion. More precisely, the translational fastener 56is moved toward the proximal end 74 of the fixation member 52 such thatthe threads 78 of the proximal end 74 enter the threaded bore 112 of thetranslational fastener 56. The translational fastener 56 is rotated toadvance the threaded bore 112 along the threads 78 until the flange 116presses snugly against the adjoining annular surface of the rotationallocking member 86. This effectively presses the bone apposition surface66 of the fixation portion 60 of the implant 50 against thesemispherical resection 34 of the corresponding saddle point 32.

Referring to FIG. 2, a perspective view illustrates the apparatus 10 infully assembled form on the vertebra 12. The position and orientation ofthe implant 50 are fixed with respect to the vertebra 12.Advantageously, since the orientation and position of the implant 50 areindependently locked, any subsidence of the bone around the saddle point32 will not enable the implant 50 to rotate from its desired orientationwith respect to the vertebra 12. If such subsidence occurs, the positionof the implant 50 may be stabilized with relatively simple revisionsurgery, i.e., by further tightening the translational fastener 56 or byinserting bone graft, an implant, or some other form of support into thespace between the bone apposition surface 66 and the semisphericalresection 34.

Those of skill in the art will recognize that an apparatus similar tothe apparatus 10 may be applied to the opposite side of the vertebra 12for bilateral operation. The fixation member 52, rotational fastener 54,and translational fastener 56 may be used to attach left or right,superior and/or inferior, implants to the vertebra 12. In alternativeembodiments (not shown), similar components to the components 52, 54,and 56 may even be used to secure nested fixation portions of superiorand inferior implants to a single saddle point 32. In yet otheralternative embodiments, such similar components may be used to secureother types of implants to the vertebra 12 besides facet joint implants,including but not limited to artificial discs, posterior rod fixationsystems, dynamic stabilization systems, and the like (not shown).

Referring to FIG. 3, a cephalad, section view illustrates the apparatus10 in fully assembled form on the vertebra 12. As mentioned previously,the fixation member 52 has a distal end 118 with threads 120 that engagethe interior of the corresponding pedicle 20. Another potentialadvantage to independent rotational and translational locking of theimplant 50 is that the purchase of the threads 120 within the pedicle 20is not significantly challenged by any of the steps used to lock theorientation of the implant 50 with respect to the vertebra 12. Only theaxial force exerted by locking of the translational fastener 56 istransmitted to the interface between the threads 120 and the surroundingbone. This decreases the probability that the bone between the threads120 will fail under shear and permit the distal end 118 to pull free ofthe bone.

The present invention has particular relevance to orthopedic medicine,and more particularly to facet joint replacement. However, theprinciples, structures, and methods of the present invention may also beextended to a wide variety of other fields.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. As such thedescribed embodiments are to be considered in all respects only asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

What is claimed is:
 1. An implantable system comprising: a fixationmember; an interpositional member received over the fixation member,wherein the interpositional member includes a tapered portion; adome-shaped member for receiving the interpositional member therein; anexpandable engagement member also received in the dome-shaped member;and a rotational locking member that engages the interpositional member,wherein rotation of the rotational locking member along theinterpositional member cause expansion of the expandable engagementmember.
 2. The implantable system of claim 1, wherein the fixationmember comprises a proximal end and a distal end and a sliding interfacetherebetween.
 3. The implantable system of claim 2, wherein the slidinginterface comprises a polygonal surface.
 4. The implantable system ofclaim 1, wherein the fixation member comprises a threaded portion and anon-threaded portion.
 5. The implantable system of claim 1, wherein theinterpositional member comprises a threaded portion above the taperedportion.
 6. The implantable system of claim 5, wherein the taperedportion is non-threaded.
 7. The implantable system of claim 1, whereinthe dome-shaped member has an upper opening and a lower opening.
 8. Theimplantable system of claim 1, wherein the expandable engagement membercomprises slits.
 9. The implantable system of claim 1, furthercomprising a translational locking member.
 10. The implantable system ofclaim 9, wherein the translational locking member comprises a flange.11. An implantable system comprising: a fixation member; aninterpositional member received over the fixation member, wherein theinterpositional member includes a tapered portion; a dome-shaped memberfor receiving the interpositional member therein, wherein thedome-shaped member is non-threaded; an expandable engagement member alsoreceived in the dome-shaped member; and a rotational locking member thatengages the interpositional member, wherein rotation of the rotationallocking member along the interpositional member cause expansion of theexpandable engagement member.
 12. The implantable system of claim 11,wherein the fixation member comprises a proximal end and a distal endand a sliding interface therebetween.
 13. The implantable system ofclaim 12, wherein the sliding interface comprises a polygonal surface.14. The implantable system of claim 11, wherein the fixation membercomprises a threaded portion and a non-threaded portion.
 15. Theimplantable system of claim 11, wherein the interpositional membercomprises a threaded portion above the tapered portion.
 16. Theimplantable system of claim 15, wherein the tapered portion isnon-threaded.
 17. The implantable system of claim 11, wherein thedome-shaped member has an upper opening and a lower opening.
 18. Theimplantable system of claim 11, wherein the expandable engagement membercomprises slits.
 19. The implantable system of claim 11, furthercomprising a translational locking member.
 20. The implantable system ofclaim 19, wherein the translational locking member comprises a flange.